JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.14, NO.3, JUNE, 2014 http://dx.doi.org/10.5573/jsts.2014.14.3.263 Analysis of Lattice Temperature in Super Junction Trench Gate Power MOSFET as Changing Degree of Trench Etching Byeong-il Lee, Jong Min Geum, Eun Sik Jung, Ey Goo Kang, Yong-Tae Kim, and Man Young Sung Abstract Super junction trench gate power MOSFETs have been receiving attention in terms of the trade-off between breakdown voltage and on-resistance [1]. The vertical structure of super junction trench gate power MOSFETs allows the on-resistance to be reduced compared with conventional Trench Gate Power MOSFETs. The heat release of devices is also decreased with the reduction of on-resistance. In this paper, Lattice Temperature of two devices, Trench Gate Power MOSFET and Super junction trench gate power MOSFET, are compared in several temperature circumstance with the same Breakdown Voltage and Cell-pitch. The devices were designed by 100V Breakdown voltage and measured from 250K Lattice Temperature. We have tried to investigate how much temperature rise in the same condition. According as temperature gap between top of devices and bottom of devices, Super junction trench gate power MOSFET has a tendency to generate lower heat release than Trench Gate Power MOSFET. This means that Super junction trench gate power MOSFET is superior for wide-temperature range operation. When trench etching process is applied for making P-pillar region, trench angle factor is also important component. Depending on trench angle, characteristics of Super junction device are changed. In this paper, we focus temperature characteristic as changing trench angle factor. Consequently, Trench angle factor don t have a great effect on temperature change. Manuscript received Aug. 25, 2013; accepted Apr. 17, 2014 Korea University E-mail : semicad@korea.ac.kr Index Terms Super junction trench gate MOSFET, conventional trench gate MOSFET, lattice temperature, trench angle I. INTRODUCTION Power MOSFETs have attracted attention since the use of the power converters and electronics has increased [2, 3]. Due to the demands for large voltage for trains and automobile, power MOSFETs have been developed to be more delicate and sensitive. Accordingly, the super junction structure has been proposed. This structure has superior characteristics in terms of on-resistance and breakdown voltage [4, 5]. In order to achieve the best electrical characteristics, temperature flow should be improved. And there are two Super Junction structures classified in fabrication process. The First one is multiepi process Super Junction MOSFET, and the second one is trench filling Super Junction MOSFET. However, the trench filling process is the simplest and more suitable for making high-aspect-ratio device 6]. In the trench filling process, by reducing the trench angle, the onresistance of trench filling Super Junction can be enhanced [7]. The on-resistance is not only changing component, but other components are also changed. In this paper, Super junction power MOSFETs are compared with conventional power MOSFETs regarding heat release. And as changing trench angle factor, we observe how heat release is changed. For fair comparison, the gates of both devices are trench structures, which reduce JFET resistance. Extracting the result is made by using TSUPREM and MEDICI simulation.
264 BYEONG-IL LEE et al : ANALYSIS OF LATTICE TEMPERATURE IN SUPER JUNCTION TRENCH GATE POWER MOSFET AS II. COMPARISON OF TEMPERATURE RISE 1. Theoretical Analysis Before measurement, trench gate super junction power MOSFETs are fabricated by the trench filling fabrication process [8]. In terms of on-resistance, the trench filling fabrication process is better than the multi-epi fabrication process [9]. Joule heating, also known as resistive heating, is one of the reasons why heat is released. Since heat release is proportional to the resistance, it is necessary to compare the resistance of both devices. The trench gate power MOSFET is shown in Fig. 1(a) with its internal resistance. This device is considered to be connected in series in the current path between the source and drain. From the top of device, the components are the source contact, source region, channel, accumulation, drift region, N+ substrate, and drain contact resistance. The drift region resistance has the largest resistance. The components of the on-resistance for the trench gate super junction MOSFET are illustrated in Fig. 1(b). This MOSFET is similar to the power MOSFET, but the resistance contributed by the drift region is reduced. Although the drift region resistance is divided into two components, it is much smaller than the drift region of the power MOSFET due to the high doping concentration in the drift region. 2. Simulation Analysis In the experiments, the breakdown voltage of both devices is 100V, and the length of the cell pitch is 2.25 µm. The other design parameters are shown in Table 1. Devices are monitored by applying heat at the heatsink. The heatsink is located at the bottom of the device. Since a certain temperature is applied to the devices, the temperature at the heatsink tends to show a similar temperature. Two devices, a power MOSFET and a super junction MOSFET, are measured at temperatures ranging from 250 K to 400 K. As shown in Fig. 2, the top area of the trench gate power MOSFET released more heat than the bottom area. This result is expected, and the junction area slightly under the top area has more heat. In Fig. 1, 300K is applied the heatsink, and the temperature difference between the top and bottom is 2.31 K. It is not mentioned specifically in this paper that if this is applied to the planar fate power MOSFET in the same conditions, the temperature difference between the top and bottom is 8.92K. This result supports the expectations of the relationship between on-resistance and heat release. Fig. 2 shows how much heat is released at the trench gate super junction MOSFET. The graph shape is very similar to the graph of a conventional power MOSFET. The top area has high temperature compared to the bottom, which is the same as in a conventional power MOSFET. But, the temperature difference between the Table 1. Design parameters of two devices (a) Conventional Power MOSFET (b) Super Junction MOSFET Fig. 1. On-resistance components in the conventional power MOSFET and super junction MOSFET. N- drift P base P pillar Design Parameter Conventional Power MOSFET Cell Pitch 2.25 Super Junction MOSFET Gate Length 1.125 0.2 Thickness Length Thickness 8.0 6.5 4.9 x 10 15 20.2 x 10 15 1.7 x 10 13 2 x 10 13-0.25-5.0-2.07 x 10 15
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.14, NO.3, JUNE, 2014 265 Fig. 3. Temperature difference of two devices. Fig. 2. Structure and temperature distribution of Conventional Power MOSFET and Super Junction MOSFET in the 10-V gate voltage, 0-V source voltage, and 20-V drain voltage. top and bottom is 0.95 K, which is smaller than in a conventional power MOSFET. The results of varying the temperature applied to the heatsink are shown in Fig. 3. With rising temperature, the temperature difference between the top and bottom is increased. III. TEMPERATURE CHANGE DEPENDING ON DEGREE OF TRENCH ETCHING 1. Simulation Analysis The trench angle is important component for making trench filling Super Junction MOSFET due to its relation to the on-resistance. A smaller trench angle is necessary for improving the on-resistance [10]. (a) 90 trench angle As shown in Fig. 4, p-pillar region is changed as decreasing trench angle. When the trench angle parameter is 90, N and P pillar meet vertically and widths are similar as shown in Fig. 4(a). As decreasing angle, P pillar region decrease. For this reason, the characteristics of device are changed. In this paper, the temperature difference between top and bottom of device is measured at various trench angles. The bias condition for simulation are 10-V gate voltage, 0-V source voltage, and 20-V drain voltage. The trench angle is changed from 90 to 89.1. In the actual process, the smallest interval of trench angle is only 0.1. (b) decreasing trench angle Fig. 4. Structure change as decreasing trench angle.
266 BYEONG-IL LEE et al : ANALYSIS OF LATTICE TEMPERATURE IN SUPER JUNCTION TRENCH GATE POWER MOSFET AS Fig. 5. Temperature difference as changing trench angle. 2. Result Analysis In the all cases of trench angle parameter, the top area of Super Junction MOSFET released more heat than the bottom area. And the junction area slightly under the top area has more heat, again. The result of simulation measurement is shown in Fig. 5. The maximum temperature difference is shown at 89.2. To see the correlation, the coefficient of correlation(r xy ) is used, r xy n ( xi mx )( yi my ) i= 1 = ( n 1) sxs y m x, m y are the average value of each, x is the trench angle parameter, and y is the temperature difference in this paper. And, s x, s y are the standard deviation of each. If r xy > 0, the result has the positive correlation. If r xy < 0, the result has the negative correlation. And If r xy is zero, this means that the result has no tendency. In this measurement, the coefficient of correlation is 0.0152. This coefficient of correlation is close to zero. Consequently, this means that there is no correlation between temperature difference and trench angle. IV. CONCLUSIONS Heat is one of the important components that should be considered for stability. According to the results of simulation, the super junction MOSFET is superior to the (1) conventional power MOSFET regarding heat. And as changing trench angle, there is no tendency of temperature change. In this paper, we analyzed a situation in which the drain voltage is at a maximum value. Devices are often used as power switches in circuits for energy conversion and management applications. To operate as switch devices, there is a moment when recombination occurs. At that time, more heat is generated than the resulting values of this paper. And we have compared only one cell pitch structure. In all package devices, we can expect that influence of the temperature gets become bigger. Also, the breakdown voltages of the power MOSFET and super junction MOSFET were fixed to 100V, 600V. However, the amount of heat released will increase when breakdown voltage exceed 100V. In order to fabricate a high-breakdown-voltage device, power MOSFETs have to be made as long-drift-region devices, which involves rising heat. ACKNOWLEDGMENTS This work was supported by the IT R&D program of the MKE/KEIT. [1003571, Development of High Voltage/ Current Power Module and ESD for BLDC Motor] and by the 2011 IT-SOC program of the MKE. REFERENCES [1] B. J. Baliga: Fundamentals of Power Semiconductor Devices, Springer, 2009 [2] H. W. Lee, et al, Study on Design of 60V TDMOSFET for protection circuit Module, J.KIEEME, Vol. 25, No. 5, pp.340-344, 2012[3] [3] S. Kyoung, et al, A Novel Trench IGBT with a Deep P+ layer beneath the Trench Emitter,, IEEE Electron Device, Vol. 30, No. 1, pp.82-84 [4] Jong Min Geum, et al, A study on the Electrical Characteristic of 600V super junction MOSFET, The Korean Institute of Electrical and Electronic Material Engineers(KIEEME), Vol. 25, No. 5, pp. 288, 2012 [5] Yu Seup Cho, et al, Electrical Characteristic Improvement of power MOSFET with Single Floating Island Structure, The Korean Institute of Electrical and Electronic Material
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.14, NO.3, JUNE, 2014 267 Engineers(KIEEME), Vol. 25, No. 1, pp. 284, 2012 [6] S. Iwamoto, et al, Above 500V Class Superjunction MOSFETs fabricated by deep trench etching and epitaxial growth, ISPSD 05. the 17th International Symposium, pp.31-3 [7] Fujihira, T, et al, Simulated superior performances of semiconductor superjunction devices, Proc. of the ISPSD 98, Kyoto,1998,pp.423-426 [8] H. Ninomiya, Y.Miura and K Kobayashi, Ultralow On-resistance 60-100 V Superjunction UMOSFETs Fabricated by multiple Ion- Implantation, Proc. ISPSD, pp.177-180,2004 [9] Pravin N. Kondekar, et al, Static Off state and Conduction State Charge Imbalance in the Superjunction Power MOSFET, TENCON Vol.4(2003),pp.1455-1458 [10] T.Minato, T.Nitta, A.Uenisi, M.Yanao, M.Harada and S.Hine, Which is cooler, Trench or Multi- Epitaxy, The Korean Institute of Electrical and Electronic Material Engineers(KIEEME), Proc. ISPSD2000(2000), pp73. Byeong-il Lee received a bachelor s degree in Electrical Engineering from Korea University, in 2013. He is currently a graduate school student in the Department of Electrical Engineering at Korea University, and he is studying semiconductor field. Especially, he is interested in Power semiconductor device and silicon carbide device. Jong Min Geum received a bachelor s degree in Electrical Engineering from Korea University, in 2011. He is currently a graduate school student in the Department of Electrical Engineering at Korea University, and he is studying semiconductor field. Especially, he is interested in Power semiconductor device and silicon carbide device. MapleSemi Co. Eun Sik Jung received Ph.D. in Electrical Engineering from Korea University. His research interests include readout integrated circuit for uncooled IR detector and highvoltage power devices. Now, He is the chief executive officer in Ey Goo Kang received M.S and Ph.D. degree in Electrical Engineering from Korea University. He is currently the professor in dept. of photovoltaic Engineering at Far East University. His research interests are Smart Power Devices and Power IC. Yong Tae Kim received the Ph.D from the Korea Advanced Institute of Science and Technology and has been working as a principal scientist in the Semiconductor Materials and Devices Laboratory, Korea Institute of Science and Technology, Seoul, Korea, since 1981. He is now Director General, National program of commercialization of nano process equipment and Vice President of the Korea Society of semiconductor and display equipment. Man Young Sung received the B.S., M.S. and Ph.D. degree in Electrical Engineering from Korea University, in 1977 and 1981, respectively. He is currently Professor in the Department of Electrical Engineering at Korea University. He has been with the University of Illinois at Urbana-Champaign as Associate Professor in 1986, and as Visiting Professor in 1997, and the Royal Institute of Technology (Sweden) where he served as Visiting Scientist in 1991.